In a nuclear medicine imaging system that is capable of coincidence imaging, a key issue is how to identify coincidence events with high accuracy. Coincidence events may be detected by applying scintillation event based trigger pulses from each of two opposing gamma camera detectors to a timing circuit. The timing circuit determines whether two pulses, one from each detector, both occurred during a period of time known as a "coincidence timing window". A key design concern is to control the duration of the coincidence timing window. Variations in the duration of the coincidence timing window may cause inaccuracies in the images generated by the system.
With the above-mentioned approach, the duration of the trigger pulses that are input to the timing circuit may dictate the duration of the coincidence timing window. Consequently, control of the duration of the trigger pulses becomes critical. The trigger pulses, which represent scintillation events, tend to have durations of only a few nanoseconds. The electronics which generate and detect such pulses include inherent propagation delays in this same range and may also include various additional delays that have been included in the system by design. Accordingly, the delays of the electronics should have extremely small tolerances (errors), to avoid variations in the coincidence timing window. Even small variations in the delays in the electronic components may cause a noticeable variation in the width of the timing pulses.
Accordingly, there is a need to provide more precise control of the width of trigger pulses generated in a gamma camera system capable of coincidence imaging. More particularly, there is a need for more precise control of timing circuit delays in such a system.